Not applicable.
The present invention generally relates to chucks for holding workpieces. More particularly, this invention relates to a pull-to-close collet chuck for use with lathes equipped with servo-type bar loaders, in which the chuck provides for dead length positioning by preventing movement of bar stock during closing of the collet.
Chucks are often used on lathes, screw machines and other machines for holding workpieces during machining operations. In lathes used to machine bar stock, chucks serve to mount workpieces to the rotating spindle of the lathe and accurately align the workpiece with the cutting tool. Chucks may be equipped with collets which, as is well known in the art, are roughly tubular-shaped with equiangularly-spaced slots in one end to delineate multiple resilient fingers capable of gripping a workpiece disposed within the bore of the collet. Servo-type bar loaders enable bar stock to be automatically fed through a collet chuck, with the axial position of the bar stock often being established by a stop provided by the lathe. Dead length positioning (also known as true position, true length positioning, and fixed length positioning) chucks eliminate the requirement for a stop on the lathe, instead relying on a single stop associated with the bar loader. However, for accurate axial positioning, a dead length positioning chuck must be capable of preventing movement of the bar stock during closing of the collet. In U.S. Pat. No. 3,434,730 to Smrekar, a dead length positioning collet chuck is disclosed in which a sleeve surrounds the end of a collet in which the resilient fingers are formed. The collet is fixed within the chuck, while the sleeve is adapted to be axially actuated relative to the sleeve by a cam and roller assembly. The sleeve and the collet end have cooperating camming surfaces so that upon axial movement of the sleeve into engagement with the collet end, the resilient fingers of the collet are compressed inward to chuck or hold bar stock within the collet.
While collet chucks of the type taught by Smrekar have been successfully used for many years, further improvements in collet chucks are desired, particularly where greater gripping forces are necessary to support larger bar stock.
The present invention provides a pull-to-close collet chuck for use with lathes equipped with servo-type bar loaders. Similar to prior art chucks used with servo-type bar loaders, the chuck is configured to be coupled to a lathe drawtube and translates linear movement of the drawtube to a work-gripping action at the collet. The chuck provides for dead length positioning by preventing movement of bar stock as the collet is closed, eliminating the need for a stock stop on the lathe. Instead, bar stock is held securely against a stop on the bar loader during the closing phase of the chuck. The chuck is constructed to promote the strength, rigidity and long wear life of its internal components.
The chuck of this invention generally includes a housing with a bore therein that defines a longitudinal axis. A collet is disposed within the bore of the housing, and is configured to include a first portion, a collapsible second portion, and a bore that is substantially coaxial with the bore of the housing. A ramp body is also disposed within the bore and surrounds the first portion of the collet, with the collet being removably secured within the ramp body by any appropriate means. The ramp body is formed to include integral means for securing the ramp body to the housing. The ramp body defines has a ramp surface that is substantially transverse to the longitudinal axis of the housing. A piston surrounds the collet and has a bearing surface adjacent the ramp surface of the ramp body. The piston is longitudinally reciprocable within the bore by a suitable actuating means. Also within the housing is a wedge located between the ramp surface of the ramp body and the bearing surface of the piston. According to the invention, the wedge differs from ball and roller bearings conventionally employed in collet chucks by having multiple bearing surfaces that provide surface-to-surface contact (as opposed to point or line contact provided by ball and roller bearings, respectively) through which the work gripping action is transferred to the collet. Two of the bearing surfaces cam against the ramp surface of the ramp body and the bearing surface of the piston, so that the wedge moves radially inward toward the collet when the piston moves toward the ramp body. Finally, the chuck includes means engaged with a third bearing surface of the wedge for causing the second portion of the collet to collapse radially inward to grip the workpiece when the wedge moves radially inward toward the collet.
The chuck of this invention is extremely rugged in terms of the strength and rigidity of its internal structure, including the manner in which the ramp body is held fast within the housing and the configuration of the wedge, whose bearing surfaces allow for better force distribution with the mating surfaces of the piston and ramp body.
Other objects and advantages of this invention will be better appreciated from the following detailed description.